1. Field of the Invention
The present invention relates to a failure diagnosis method and failure diagnosis device for an evaporated fuel treating unit and in particular, to a technology for making a determination on the leak of fuel vapor.
2. Description of Related Art
The evaporated fuel treating unit is a unit for preventing evaporated fuel produced in a fuel tank from being discharged into the atmosphere. A combined body of structural members including the fuel tank, a canister, a purge passage, and a purge control valve forms a closed space when the foregoing purge control valve is closed. This closed space is called an evaporation system. It is desired to mount a failure diagnosis device for determining whether or not evaporated fuel leaks from the evaporation system. Hereinafter, the failure diagnosis device and it's function may be referred to as leak check.
JP-A-5-272417, and U.S. Pat. No. 5,146,902 discloses a method in which pressure in the evaporation system is increased by a pump and then the state of decrease in the pressure in the evaporation system is measured at a specified time set previously to determine the state of leak. However, according to this method, the volume of a space to be pressurized is varied by the amount of remaining fuel and hence the rate of decrease in the pressure is also varied, so that it is possible to detect whether or not leak occurs but impossible to correctly detect the magnitude of the leak. Further, since the state of decrease in the pressure is varied also by differences in the atmospheric temperature and the properties of the fuel, it is impossible to determine the state of the leak sufficiently correctly if no correction is made. For example, the atmospheric temperature and the properties of the fuel affect the amount of evaporated fuel at a certain temperature. In order to grasp the state of the leak correctly, it is thought to correct the state of the leak by parameters affecting the determination such as the amount of remaining fuel but this makes the determination complex and hence increases cost. On the other hand, if stricter conditions for allowing the leak check are become stricter, it is impossible to achieve an essential object of ensuring the frequency of determinations.
On the other hand, JP-A-10-90107 discloses a method in which a pump is driven until operating characteristic values such as current, voltage, the number of revolutions are saturated and the saturated operating characteristic values are compared with the base values to determine the state of leak. However, according to this method, the pump is driven until the operating characteristic values are saturated and hence time to drive a pump is elongated, which degrades fuel consumption. Moreover, it is necessary to use a long-life pump or to increase the frequency of replacements of the pump and hence to increase cost.
Still further, JP-A-11-351078 discloses a method of using a base orifice. Variations in pressure in the evaporation system in this technology are shown in FIG. 19. The state of decrease in the pressure in the evaporation system from the time when the pressure is increased to a specified pressure P0 to the time when a previously set time T elapses is measured in a case where leak occurs at an orifice as a base leak point and in a case where leak does not occur. The pressure changing state may be measured as pressure differences P1, and P2. The amount of change in the pressure P2 caused by a leak hole as a failure is compared with the amount of change in the pressure P3=P1−P2 caused by the orifice as the base leak hole thereby to cancel effects produced by the amount of remaining fuel, atmospheric temperature, fuel properties, and the like.
However, in this method, when the amount of remaining fuel is extremely large, that is, the volume of the space to be pressurized is extremely small, as shown in FIG. 20, the rate of decrease in the pressure is increased to make the pressure zero, atmospheric pressure, that is equal to the pressure outside the evaporation system before the foregoing time T elapses. This is not the proper amount of change in pressure. On the other hand, when the amount of remaining fuel is small, the rate of decrease in the pressure is decreased to make it impossible to produce the sufficient amounts of change in pressure P1, P2 and hence to produce a sufficient detection accuracy. For this reason, there is a fear that correct determination can not be made on the state of leak. If the pressure in the evaporation system is sufficiently increased, the state of leak can be correctly determined but there are raised a problem that a fuel tank and the like need to have sufficient resistance to pressure and a problem that the capacity of a pump for increasing pressure needs to be increased. These problems can not be easily solved.
Still further, JP-A-11-351078 discloses a method of measuring time required for the pressure in the evaporation system to decrease by a specified amount of pressure drop. The measurement of time can be easily performed with higher accuracy than the detection of pressure. However, the determination method based on the amount of changes in the pressure P1, P2 can not be used for the determination based on the required time. For this reason, in order to put the method of leak check utilizing the required time into practical use, some improvements need to be made that are not disclosed in JP-A-11-351078.
The present invention has been made in view of the above circumstances. It is the object of the invention to provide a failure diagnosis method and a failure diagnosis device of an evaporated fuel treating unit by which a correct leak check can be performed regardless of the amount of remaining fuel and practically.